The objective of cryptographic key distribution is to enable two users who share no secret information initially, to agree on a key (e.g., a string of random bits), which enables them to form and transmit encrypted messages, while limiting a potential eavesdropper to information about a vanishingly small fraction of the key. The key may be used as a “one time pad,” to encrypt a message. Alternately, or in addition, the key may be used in one or more cryptographic algorithms, such as DES, Triple-DES, IDEA, or AES/Rijndael. Additional applications include using the key in one or more electronic message signature, validation, or authentication schemes.
To facilitate description of various key distribution techniques, this specification employs the convention of using “Alice” to designate a key information sender, “Bob” to designate a key information receiver, and “Eve” to designate an eavesdropper. Under this convention, messages may be sent both from Alice to Bob and from Bob to Alice. Thus, Bob may transmit key information to Alice, in addition to Alice transmitting key information to Bob.
Advancing research and experimental investigation of fundamental problems of quantum mechanics have led to a methodology for secure communications: quantum cryptography. At the heart of this technique lies the distribution of a cryptographic key whose security is guaranteed by the principles of quantum mechanics. With Quantum Key Distribution (QKD), two users, Alice and Bob, agree on a secret key using a communications channel capable of transmitting sequences of quantum states. Due to the Heisenberg uncertainty principle, it is not possible to accurately monitor these sequences without disturbing them in a way that can be readily detected by the legitimate users of the channel. After the transmission of a quantum sequence between Alice and Bob across this channel, Alice and Bob exchange messages through a second channel, called the public channel. Alice and Bob communicate across this public channel to assess the amount of disturbance in the transmission of the quantum sequence to determine the extent of noise and eavesdropping. If it is determined statistically that eavesdropping has not occurred, Alice and Bob select a body of information from the transmitted sequence in order to form a key that, with a high probability, is known only to the two of them and not to an eavesdropper, Eve. In QKD, the private quantum channel is combined with a public classical channel to create a conduit with high overall security. A variety of QKD protocols are known, such as BB84, B92, 4+2, Six State, Ekert scheme, and Goldenberg/Vaidman class.
Two photons quantum-mechanically entangled together are referred to as an entangled-photon pair, or biphoton. Traditionally, the two photons comprising a biphoton are called “signal” and “idler” photons. The designation of which photon is referred to as “signal” and which is referred to as “idler” is arbitrary. The photons in an entangled photon pair have a connection between their respective properties. Measuring properties of one photon of an entangled-photon pair determines properties of the other photon, even if the two photons are separated by a distance. As understood by those of ordinary skill in the art and by way of non-limiting example, the quantum mechanical state of an entangled-photon pair cannot be factored into a tensor product of two individual quantum states.